Keywords: squid, scallop, jet propulsion, hydrodynamic efficiency, Antarctica, ontogeny, scaling Abstract: Among multicellular animals, jet propulsion is nature’s simplest (and arguably its oldest) form of aquatic locomotion. Any flexible, hollow body girdled by circumferential muscle fibers, can, by expelling fluid through an orifice, produce thrust and thereby swim. Despite the fundamental simplicity of this locomotory mechanism, aspects of its realization in nature continue to provide insight into the physiology, ecology, and evolution of a wide variety of animals. In this talk, I report on two mollusks that use jet propulsion. The Antarctic scallop is one of only a few bivalves that can swim. Like its temperate and tropical cousins, it claps its shells together to expel a jet of water sufficiently powerful to lift both its internal organs and its dense calcium-carbonate shell off the seafloor. But the Antarctic scallop must perform this feat in water at -1.86 degrees C, a temperature at which muscle power is reduced and water’s viscosity is 1.43 times that of tropical water. Shell mass in the Antarctic scallop is much reduced relative to tropical scallops, but muscle mass is reduced even more. The only net advantage evident in Antarctic scallops is the increased resilience of the “spring” that opens the shell, suggesting that even slight increases in hydrodynamic efficiency can be selected during evolution. Increases in hydrodynamic efficiency may also play an important role in squid locomotion. Unlike most jet propulsors (e.g., jellyfish, salps, clams), squids can actively control the size of the orifice through which water is expelled. Appropriate narrowing of the jet orifice during mantle contraction can boost the efficiency of both the hydrodynamics of propulsion and the contraction of muscle. This potential increase in efficiency may be most important in small juvenile squid, for whom jet propulsion is otherwise very inefficient.